The invention relates to a servocontroller which controls a system comprised of energy-storing elements, the controller being operative for controlling the controlled system by generating a pulse-duration-modulated actuating signal which actuates a two-position control element whose positions have a substantially delay-free effect upon the controlled system itself, but only a delayed effect upon the value of the output variable of the controlled system, be it position, temperature, pressure, velocity, or the like.
Some controlled systems exhibit the characteristic that the value of the output variable of the controlled system tends to decrease in response to external factors. For example, in the case of a heat-storing controlled system, if heat is not supplied to the controlled system, loss of heat occurs, and the output variable of such controlled system (i.e., temperature) will tend to decrease. When this is the case it is a common expedient to control the contolled system by means of a servocontroller which is operative only for initiating and terminating the supply of energy to the controlled system, it being unnecessary for the controller to remove energy from the controlled system, since loss of energy from the control system will occur by itself, when the controlled system is not receiving energy under the action of the controller. A thermostat-type on-off controller is a familiar example of this type of controller. However, there are many other controllers of this type.
Such controllers are operative for effecting intermittent supply of energy to the energy-storing controlled system at an average rate sufficient to counteract the continual loss of energy which occurs even in the steady state.
If such a servocontroller is to perform well, it is theoretically desired that the supply of energy to the energy-storing controlled system should be automatically varied in immediate response to changes in the energy input rate actually required to maintain the output variable of the controlled system at the desired value; i.e., the servocontroller should ideally be a direct (delay-free) controller immediately responsive to undesired changes occurring within the controlled system.
However, because of the presence of energy-storing elements within the controlled system, the initiating and termination of the supply of energy to the controlled system by the controller must take into account the dynamic behavior of the controlled system, namely the behavior of the controlled system during an automatic servocompensation operation, thereby complicating the realization of ideal control action.
In this connection, particular care must be taken when the controlled system exhibits substantial dead time. For example, if the output variable of the controlled system has the desired value and the controlled system is in fact in the proper state of energization, a departure from such proper state of energization will result in a corresponding change in the value of the output variable of the controlled system, thereby initiating renewed supply of energy to the controlled system. The energization of the controlled system proceeds until a time when enough energy has been furnished; however, there may be a substantial dead time before the output variable of the controlled system reassumes the desired value.
With on-off servocontrollers the existence of a dead time of this sort has a marked effect upon overall system performance. With on-off servocontrollers, the dead time results in the termination of energy supply to the controlled system than is theoretically desirable; in theory, the supply of energy should be initiated in immediate response to the development of an energy requirement in the controlled system and should be terminated in immediate response to the successful filling of the energy requirement.
In case of such dead time, the energizing pulse will be somewhat too long, the excess in the duration of the energizing pulse being substantially constant, independent of the duration of the energizing pulse. In other words, the absolute value of the excess duration of the energizing pulse will be the same for both short-duration and long-duration pulses. Clearly, in the case of long-duration energizing pulses the excess duration will represent only a small percentage of the total pulse duration; however, in the case of short-duration energizing pulses, the excess duration will represent a much larger and possibly very significant percentage of the total pulse duration.
The longer-duration energizing pulses are associated with greater descrepancies between the desired and actual values of the output variable of the controlled system; the shorter-duration energizing pulses are associated with smaller discrepancies between the desired and actual values of the ouput variable of the controlled system. This results in servocontroller performance different from what is usually desired. It is usually desired that the actuating or compensating signal increase with increasing system error, and decrease with decreasing system error; usually it is desired that the actuating or compensating signal have a magnitude which is substantially proportional to the controlled system error. If the servocontroller operates in an on-off manner, and if the actuating or compensating signal has the form of a pulse-duration-modulated pulse train applied to a two-position control element such as a relay switch or a solenoid valve, then the effective value of the actuating or compensating signal is the ratio of the ON-time to the OFF-time of the pulse-duration-modulated pulse train.
In such an on-off controller with pulse-duration-modulated actuating signal, the dead time discussed above results in system performance which is sub-proportional (degressive). Specifically, the ratio between, on the one hand, the effective value of the actuating signal and the system error is not constant, as would be the case in an ideal proportional control system, but instead decreases as the system error increases (it is to be recalled that the "effective value of the actuating signal" has been defined above as the ratio of the On-time to the 0FF-time of the pulse-duration-modulated actuating signal).
This sub-proportional (degressive) system performance results, as should be clear from the above explanation, when a proportional-acting servocontroller of the type employing a pulse-duration-modulated actuating signal to actuate a two-position control element is used to control a controlled system having a substantial dead time. Nevertheless, there are important advantages in the use of a servocontroller of the type employing a pulse-duration-modulated actuating signal. These advantages include the possibility of using a simple two-position control element in place of a continuously operable servomotor, of course in place of a simple two-position control element such as a relay switch or solenoid valve, used could be made of an electronic switch, to which the term "two-position control" element likewise applies, as used herein.
An additional advantage of an on-off servocontroller which generates a pulse-duration-modulated actuating signal to actuate a two-position control element lies in the fact that, automatic servocompensation operations can often be performed very quickly, because the servocontroller connects the controlled system directly to the energy source, so as to always furnish energy at the maximum possible rate so long as the source remains applied. Another advantage is that the pauses between the successive energizing pulses give the controlled system time to react to the supply of energy. The reaction of the controlled system during the pause following an energizing pulse will actually tend to have an effect upon the duration of the next-following energizing pulse; this is desirable because it results in more accurate control. Also, as is well known, the existence of the energy-storing elements in the controlled system combined with properly selected frequency ranges for the pulse-duration-modulated actuating signal (namely frequency ranges that are sufficiently high) results in a system performance which closely approximates to that of servocontrollers of the continuously-acting type.
As discussed at some length above, the controlled system may exhibit a substantial dead time. An example of this would be the following: A sealed container containing a plurality of chemical reactants which are to be precisely maintained at a predetermined constant temperature, with the sealed container including an electrically energizable heat-dissipating element such as a heating coil, and the container further including a temperature-detecting element also located in the container spaced from the heating coil so as to be responsive not so much to the temperature of the heating element but instead, as is desired, responsive more to the average temperature of the substances to be maintained at the constant temperature. If, due to loss of heat from such system, the average temperature of the liquid in the container decreases, such decrease in the average temperature is detected by the temperature-responsive transducer almost immediately. If in response to detection of the undesired temperature decreases a pulse of current is passed through the heating coil, the temperature of the heating coil may rise almost immediately to a greatly increased temperature, and after elapse of the dead time the heating coil and the heated fluid come into thermal equilibrium, perhaps restoring the liquid temperature to the desired value, which fact is then detected by the temperature-responsive element in the fluid. In other words, the first heating pulse just mentioned resulted in the supply of enough heat energy to the controlled system (container plus heating coil plus liquid) to eventually reduce to zero the system error; however, the aforedefined dead time elapses before the system comes into thermal equilibirum and the system error actually returns to zero.
If, to control a system such as the exemplary one just described, use is made of a direct servoncontroller of the type generating a pulse-duration-modulated actuating signal, and if the servocontroller has a porportional performance (effective value of pulse-duration-modulated actuating signal is proportional to detected system error), then the overall performance of the servocontroller and the controlled system will be degressive, as explained above. Such degressive performance requires either a large proportinal range or a substantial limitation of the range of value of the actuating signal. The result is unsatisfactory, because the steady-state system error will be too large.
It is known to overcome this disadvantage by making use of a pulse-duration-modulating servocontroller whose performance is proportional-plus-integral. The integral action time of such proportional-plus-integral servocontroller will be selected in correspondence to the dead time of the controlled system. However, since with this expedient the development of the error-compensating actuating signal is markedly delayed, the overall regulating action is sluggish. The effective value of the pulse-duration-modulated actuating signal changes only slowly, and accordingly if a system error develops the servocontroller does not then counteract such system error in the quickest possible way. Also, the dead time of the controlled system causes the change of polarity of the effective value of the actuating signal to occur too late, namely too late by the amount of such dead time, and this action can lead to excessive overshoots and excessively oscillatory performance. Also, such a servocontroller continually produces changes in the phase shift of the actuating signal, and this creates a tendency for the system to oscillate excessively. Accordingly, when such a controller is used to control such a controlled system, the proper adjustment of the operating parameters of the servocontroller is very critical, and not easy to perform. Correct selection of the parameter values and matching of the controller and controlled system performance is a costly process. Also, the possibility of incorrect design is very great.